1. Field of Invention
The invention relates to an optical head objective lens and, in particular, to an optical head objective lens that uses the near-field optics and a micro aperture generated via the surface plasmon effect to minimize the size of the light spot.
2. Related Art
The task of an optical head objective lens is to generate a light spot with a small diameter. The smaller the diameter is, the higher the storage capacity of the optical disk is. The diameter of the light spot is inversely proportional to the numerical aperture (NA) of the objective lens. It is known that NA=n* sin θ, where n is the index of refraction at the point where the light beam converges and θ is the angle between the outermost ray and the optical axis of the incident beam. Therefore, one can try to increase the NA of the beam to minimize the light spot. The conventional optical head objective lens uses the far-field design whose limit NA value is 1. Using the near-field optics principle can easily make the NA greater than 1.
The basic structure of a conventional near-field optical head objective lens is shown in FIG. 1. It includes the combination of a convergent lens 10 and hemispheric lens 11. When the convergent lens 10 converges the laser beam to the lower surface of the hemispheric lens, the beam is totally reflected as a result of an appropriate combination of the index of refraction n of the hemispheric lens and the incident angle. When the disc 12 is close to the hemispheric lens 11, the light totally reflected from the hemispheric lens 11 will propagate its energy to the underneath disc in the form of evanescent waves when the distance is properly adjusted. As the distance changes, the energy carried by the evanescent waves also varies, resulting in 0 and 1 signals. The NA value is n*sin θ, the size of the light spot is about λ/NA, where λ is the wavelength. Suppose, λ=660 nm, n=2.0, and sin θ=0.6, the diameter of the light spot thus generated is about 550 nm. Using the full width at half maximum of the energy spectrum, one obtains a diameter of about 280 nm. The increase in the NA value of such objective lenses is very limited. Another type of near-field optical head objective lens is shown in FIG. 2. A super-spherical lens 14 replaces the original hemispheric lens 11. The NA value of such objective lenses is n2*sin θ. Using the same set of values as before, we obtain NA=2.4 and the light spot size is about 280 nm. Using the full width at half maximum of the energy spectrum, one obtains a diameter of about 140 nm. The diameter is shrunk by a factor of about 2. However, the precision requirement in manufacturing such super-spherical structure 14 is very high. Therefore, there are difficulties in mass production.
To solve the problems in minimizing the light spot and manufacturing difficulties, the structure shown in FIG. 3 was proposed. A metal film 18 is coated onto the lower convergent surface of the hemisphere of a conventional near-field objective lens. A micro aperture 19 is further opened at the place where the light spot becomes focused on the metal film 18. The light spot size is then directly related to that of the aperture. If one can control the aperture size below 100 nm, the light spot should have a size also of order 100 nm. However, one difficulty is the alignment between the micro aperture and the laser beam. For this problem, a structure as in FIG. 4 was proposed. A micro aperture array 24 is formed on the metal film to lower the difficulty in alignment. With regard to the high-density data storage technology, Junji Tominaga et al. proposed the concept of a super-resolution near-field structure (Super-RENS) in April 1998. As shown in FIG. 5, when the laser beam is converged by the convergent objective lens 10, it is sent to an optical disc 31 with the Super-RENS. Once the light touches the film 31-b that generates the surface plasmon effect, only beams whose central energy is greater than a particular threshold intensity can pass through. The intensity of the penetrated beam will further be amplified. Moreover, the light spot size is not sensitive to the wavelength. Therefore, the light spot diameter on the recording layer 31-d can be minimized. It also has the advantage of a superior energy usage rate. The use of a surface plasmon film makes it possible to shrink the converged light spot diameter down to less than 200 nm.
The structure of coating a metal film on the lower convergent surface of the hemisphere of a conventional near-field objective lens has a very low energy usage rate because of the small aperture and the low penetration rate. Although coating a homogeneous surface plasmon film on the Super-RENS optical disc can solve the problem in energy use, there is still technical difficulty in making a large-area surface plasmon film (usually with a diameter of 120 mm). Besides, other problems in making near-field objective lenses (such as using a material with a large index of refraction, a short-wavelength light source, or positioning when using metal micro apertures) are limitations derived from minimizing the light spot diameter.